ATLANTA -- A Delta jet narrowly averted disaster Monday when it mistakenly landed on a taxiway instead of a runway at Hartsfield-Jackson International Airport.

Pilots use taxiways to reach the runways. They are often lined with planes waiting to pull onto a runway for takeoff.

When Delta flight 60 from Rio de Janeiro landed on the taxiway just after 6 a.m. on Monday, the taxiway was empty. Landing at about 160 mph, if the Boeing 767 had hit another plane, the results could have been catastrophic.

The FAA's Kathleen Bergen said the Delta flight was cleared to land on runway 27-right. Instead, the pilot landed on taxiway-M, which runs alongside the 12,000 ft. runway.

Runways are lined with white lights; taxiways are lined with blue lights.

The FAA is investigating the incident.

"Pilots are trained to land on the runway," Bergen says. "Taxiway landing is not appropriate, so we will be investigating it very thoroughly in determining why that happened.

"We did receive a report that there was a medical emergency on board the aircraft when the landing occurred," she says, "but the pilot still is required to follow all proper procedures and land on the runway."

Such landings are rare, but have happened.

"I can't think of any prior occasion in Atlanta," says Bergen. "But we have had patterns of taxiway landings at other airports in the southeast."

One southern airport, in particular, has had its share of taxiway landings.

"We've had a number of these cases occurring at Palm Beach International Airport," she says. "That is causing us to take a look at the airport layout."